Fix MSIX detection in Register-MdpToClaudeDesktop

Detect MSIX install by checking for the package directory existence
instead of the config file, so the correct path is used even when
claude_desktop_config.json has not been created yet.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: yotsuda

Module/MarkdownPointer.psm1

@@ -338,8 +338,14 @@ function Register-MdpToClaudeDesktop {
     $command = Get-MarkdownPointerMCPPath
 
     # Determine config file path per platform
+    # MSIX (Microsoft Store) installs virtualize %APPDATA% under Packages\
     $configPath = if ($IsWindows) {
-        Join-Path $env:APPDATA 'Claude\claude_desktop_config.json'
+        $msixDir = Get-ChildItem "$env:LOCALAPPDATA\Packages\Claude_*" -Directory -ErrorAction SilentlyContinue | Select-Object -First 1
+        if ($msixDir) {
+            Join-Path $msixDir.FullName 'LocalCache\Roaming\Claude\claude_desktop_config.json'
+        } else {
+            Join-Path $env:APPDATA 'Claude\claude_desktop_config.json'
+        }
     } elseif ($IsMacOS) {
         Join-Path $HOME 'Library/Application Support/Claude/claude_desktop_config.json'
     } elseif ($IsLinux) {